Monofluoromethylthioacetic acid (FCH.sub.2 SCH.sub.2 COOH) or difluoromethylthioacetic acid (F.sub.2 CHSCH.sub.2 COOH), or its derivative at the carboxy group can be prepared e.g. by treating a thioglycolic acid ester with monolfuorohalomethane or difluorohalomethane in the presence of a base if required followed by a conventional modification of the produced ester e.g. to make an acid by hydrolysis, a salt by neutralization, a halide with a halogenating agent, and from acid halide or anhydride an ester with an alcohol, or an amide with an amine. These compounds are useful intermediates in the production of medicinal and agricultural chemicals.